Many of the plants we grow begin life as seeds (exceptions are ferns and mosses, which develop from spores, and plants grown from cuttings or by other means of asexual reproduction). A seed is an amazing object, containing within it a rudimentary plant (the embryo) and a supply of food used both to start the embryo growing and to feed the seedling until it can manufacture its own food through photosynthesis. Seeds of some plants are viable (capable of germination) for only a brief time; others remain sound for many years, sometimes even for centuries, helping to ensure the survival of their species.
Seeds cannot germinate until certain favorable environmental conditions are met. These include adequate moisture, a preferred temperature, and, for most species, a loose-textured soil that provides oxygen to the sprouting seed. Gardeners supply these conditions when planting seeds in potting mixes or well-prepared seedbeds.
Some seeds have additional requirements for germination. For example, seedlings of plants from cold-winter areas are most likely to survive if they sprout in spring, after winter is past: were the seeds to sprout immediately after ripening in fall, the seedlings would be killed by freezing weather. To ensure the necessary delay in germination, such seeds will sprout only after they are first moistened by fall rains, then exposed to a period of low temperatures. Called stratification, this process occurs naturally outdoors--but gardeners can mimic it by placing the seeds between layers of damp paper towels in a plastic bag and storing them in the refrigerator for a month or two. Other kinds of seeds may require nicking, grinding, or scarifying to break their hard seed coats; still others may need light, darkness, or even intense heat to sprout.
Seed size varies with the kind of plant. The range is wide, from the very large seeds of a coconut palm, down through sunflower and mustard seeds, to the very tiny, almost dustlike seeds of begonias. A seed's size is proportional to the amount of stored food it contains and thus to the maximum allowable planting depth. Small seeds, for example, have small food reserves; if planted too deeply, they will use up that food and die before the cotyledons reach the soil surface. Seed packets give directions on planting depth. As a rule of thumb, it is safest to bury a seed no deeper than its length.